2.1 Nervous, Muscular, Skeletal, Cardiorespiratory, and Endocrine Systems

The Human Movement System

NASM's CPT7 textbook frames anatomy around function. The Human Movement System (HMS), also called the kinetic chain, combines three systems that produce and control motion: the nervous system, the skeletal system, and the muscular system. The CPT exam rarely asks for an isolated anatomy fact; it asks what is happening inside a moving client, so learn each system as a training-decision tool rather than disconnected vocabulary.

The nervous system is the master communication network. It has two main divisions. The central nervous system (CNS) is the brain and spinal cord — it integrates information and issues commands. The peripheral nervous system (PNS) is everything outside the CNS: the cranial and spinal nerves that connect the CNS to the rest of the body. The PNS itself splits into a sensory (afferent) division, which carries information toward the brain, and a motor (efferent) division, which carries commands out to muscles and glands.

Neurons, Motor Units, and Mechanoreceptors

The functional unit of the nervous system is the neuron, built from a cell body, dendrites (receive signals), and an axon (sends signals). A motor unit is a single motor neuron plus all the muscle fibers it controls. When the neuron fires, every fiber in that unit contracts — the basis of the all-or-none principle covered in Section 2.4.

Proprioception — the body's awareness of its position and movement — depends on sensory receptors called mechanoreceptors. The two most exam-relevant are:

These two receptors anchor NASM's flexibility logic: foam rolling and static stretching exploit GTO-driven autogenic inhibition, while active stretching uses spindle-driven reciprocal inhibition of the antagonist (detailed in Section 2.5).

Skeletal and Muscular Systems

The skeletal system provides the levers and the framework that muscles pull on. The adult skeleton has roughly 206 bones, organized into the axial skeleton (skull, vertebral column, rib cage — about 80 bones, providing protection and a central axis) and the appendicular skeleton (shoulder girdle, pelvic girdle, and the limbs — about 126 bones, producing locomotion). Bones also store minerals (calcium, phosphorus) and house red marrow that produces blood cells. Where bones meet, joints allow or restrict motion (Section 2.2).

The muscular system is the force generator. Skeletal muscle attaches to bone via tendons; the muscle's fixed proximal attachment is the origin, and the movable distal attachment is the insertion. When a muscle contracts, it pulls the insertion toward the origin. Connective tissue layers — endomysium (around each fiber), perimysium (around fascicles), and epimysium (around the whole muscle) — transmit force into the tendon.

Cardiorespiratory and Endocrine Support Systems

The cardiorespiratory system is the engine that supplies oxygen and removes waste. It combines the cardiovascular system (heart, blood, and blood vessels — arteries carry blood away from the heart, veins return it) and the respiratory system (airways and lungs, where gas exchange occurs in the alveoli). Two numbers the exam tests:

• Stroke volume (SV): blood ejected per heartbeat.
• Cardiac output (Q): total blood pumped per minute, where Q = heart rate × stroke volume.

When exercise intensity rises, the body increases heart rate and stroke volume to raise cardiac output and meet the muscle's oxygen demand (VO2). VO2max is the highest rate of oxygen uptake achievable at maximal exertion — a key measure of aerobic fitness.

The endocrine system uses hormones as chemical messengers that regulate metabolism, adaptation, and the stress response. High-yield hormones: cortisol (catabolic stress hormone, mobilizes energy), testosterone and growth hormone (anabolic, support tissue repair and growth), and insulin (drives glucose into cells). Chronic overtraining or poor recovery can keep cortisol elevated, blunting adaptation — a reason trainers manage volume and rest. Endocrine disorders are outside a CPT's scope; suspected issues are referred to a physician.

Connecting the Systems to Training Decisions

The reason NASM groups these systems is that no single one acts alone during exercise. Consider a single set of barbell squats. The nervous system reads the load through the muscle spindles and Golgi tendon organs, recruits the appropriate motor units, and adjusts firing to keep the client balanced. The skeletal system provides the rigid levers, and the joints channel the forces along safe paths. The muscular system generates the tension that moves the levers.

Meanwhile the cardiorespiratory system raises heart rate and stroke volume to deliver oxygen to the working muscles, and the endocrine system releases hormones that mobilize fuel and, over time, drive the adaptation that makes the client stronger.

This integration is why NASM tests systems in the context of decisions rather than as trivia. A few practical links the exam expects a trainer to make:

• A client with poor proprioceptive feedback (weak sensory input) benefits from stabilization work on progressively less stable surfaces to challenge the nervous system.
• A deconditioned client fatigues quickly because limited cardiac output cannot meet oxygen demand, so the trainer starts at lower intensities and builds aerobic base first.
• A client under chronic life stress with elevated cortisol may recover poorly, so the trainer adjusts volume, prioritizes sleep guidance, and avoids excessive high-intensity work.

Understanding the systems as one coordinated whole — the Human Movement System supported by its oxygen-delivery and hormonal engines — is the foundation for every assessment, program-design, and progression decision later in the certification.